Fiber laser devices can produce a small beam spot of light having high focusing performance and high power density, and process in a noncontact manner. Accordingly, fiber laser devices are used in various fields such as the laser processing field and the medical field. A fiber laser device includes an amplification optical fiber having a core coated with a clad and doped with an active element for amplifying light. However, it is known that skew mode propagation may occur in the amplification optical fiber, where part of pumping light propagates only through the clad without being absorbed by the active element and does not contribute to amplification of light. One known technique for preventing such skew mode propagation is to form the clad of the amplification optical fiber to have a cross-section of a non-circular shape such as a D-shape or a polygonal shape.
Patent Document 1 listed below discloses an amplification optical fiber in which a clad is formed to have a polygonal cross-section as mentioned above. According to Patent Document 1, in connecting the amplification optical fiber having a clad of a polygonal shape to another optical fiber, cores of both optical fibers to be connected are butted against each other under side view observation, where accurate positions of the cores may not be visually obtained depending on the shape of the clad of the amplification optical fiber. Patent Document 1 also states that accurate positions of the cores can be visually obtained by side view observation if the clad of the amplification optical fiber has a substantially square shape.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-229617